Add target-latency and current-latency to debug view

[ryanmccartney]

📺 What

Adds MSE target latency and current latency reporting to the debug menu. Useful when debugging low-latency content, particularly on TV hardware where some decoder behaviours do not match what might be expected

🛠 How

Uses existing DebugTool.dynamicMetric() method to add these metrics each time a timeupdate event is called.

✅ Testing

Added underlying dash.js functions to the mockDashInstance. No additional unit tests were added.

Test Guidelines

Test engineer sign off	❌

👀 See

New fields are seen below when the MSE strategy is chosen. The values NaN and 0 are seen when VOD content is played.

The same changes are shown again, but using live low-latency content. Extra fields, along with the existing playback rate field, can be used to determine whether low-latency content is performing as expected.

♿ Accessibility [optional]

[Any accessibility features or considerations that this PR addresses should be listed here]